Caption: Retina photoreceptor cells. Artwork of the four types of photoreceptor cell found in the retina. A rod cell (pink, left) aids vision in dim light. The red, blue and green cone cells combine vision in light of those colours, allowing colour vision. Rods and cones are named for the shape of their upper sections. The upper sections carry the photochemicals rhodopsin (rod) and iodopsin (cone, three types). The retina is at the back of the eye, and light entering the eye (from bottom) hits the photochemicals and triggers a nerve impulse. These impulses are sent to the brain by nerve cells (not shown) at the photoreceptor bases. Brain processing then produces visual sensations.